I. Field of the Invention
This invention relates to pressure transducers which are specifically adapted for measuring bodily fluid pressure within a conduit.
II. Description of the Prior Art
Pressure within a conduit can be measured in a number of ways. Perhaps the most common is a mechanical gauge which is coupled through one wall of the conduit to the fluid pressure within the conduit. Inside the gauge, a needle is deflected over a scale in proportion to the pressure within the conduit. In some instances, the standard pressure gauge may be replaced with a transducer which converts pressure into an electrical signal which is then monitored. The present invention is, broadly speaking, of the latter transducer type.
Pressure measurement of fluids within a conduit occurs in numerous environments. The present invention is particularly concerned with measuring bodily fluid pressure within a conduit in a medical environment. For example, a patient's blood pressure, and/or intracranial pressure and the like may have to be continuously monitored.
One typical method of monitoring blood pressure is to measure the fluid pressure within an intravenous tube which is hydraulically coupled to the patient's vein. A catheter is inserted into the patient's vein and a plastic tube or conduit coupled to the catheter. A saline solution is drip-fed through the plastic tubing or conduit to maintain a pressure balance against the pressure within the patient's vein. The saline fluid acts as a hydraulic fluid to cause the pressure within the plastic tubing to correspond to the pressure within the patient's vein. Hence, by measuring the fluid pressure within the tubing, the patient's blood pressure will be known.
One known device for measuring the fluid pressure in the plastic tubing involves a two-part dome/sensor apparatus. The dome is typically a hollow replaceable structure which is inserted between two lengths of plastic tubing and secured thereto by Leur locks (which are fittings that receive and lock a standard size tubular fitting or tube carrying an intravenous solution such as saline), for example. The saline or hydraulic fluid fills (i) a portion of the tubing above the dome, (ii) the hollow within the dome, and (iii) the lower portion of the tubing which is coupled to the patient's catheter. The bottom wall or side of the dome includes a diaphragm. Releasably secured against the dome diaphragm is a sensor structure including a mating diaphragm. Any pressure within the dome results in displacement of the dome diaphragm which is transmitted to the sensor's mating diaphragm similarly displacing the mating diaphragm. An example of the foregoing is described in U.S. Pat. No. 4,252,126 assigned to the assignee herein, Medex, Inc., and incorporated herein by reference.
The mating diaphragm is mechanically coupled to an electrical sensor including a strain gauge or the like. The sensor produces a DC electrical signal corresponding to the displacement of the mating diaphragm, i.e., corresponding to the pressure in the patient's vein.
Strain gauges of various constructions and material are known. Strain gauges employed to monitor blood pressure utilize semiconductor materials such a ceramic or quartz-type piezoelectric materials. Pressure applied to the strain gauge causes the conductance of the material to change. To measure the change in conductance, the sensor's strain gauge is part of a Wheatstone bridge network, the output of which is a voltage proportional to the pressure. The bridge output is typically coupled to commercially available monitors which are adapted to provide medical personnel continuous readout of the patient's blood pressure.
Sensors typically must occasionally be "zeroed" or calibrated to atmospheric pressure thus adding complexity and/or operator involvement to the pressure monitoring process. Further, offsets due to the temperature of the environment in which the transducer is stored and/or operated may induce errors into the system which can be costly, albeit necessary, to correct. With respect to the two part dome/sensor system, the mating sensor structure and, especially, the strain gauge are quite costly.
A further drawback to the dome/sensor system is that medical personnel utilizing or operating the equipment must not only set up the hydraulic system discussed above, but must also connect the sensor to the dome and be certain that the connection is secure. Hence, operation steps are added which can be time consuming and subject to human error. Also, with the two-part system, the dome is replaceable but the sensor structure is reusable and thus should be sterilized between use, thereby increasing costs.
The medical environment for which this invention is most particularly adapted places severe limitations on many aspects of a transducer. The Association for the Advancement of Medical Instrumentation (AAMI) has proposed standards applicable to blood pressure transducers entitled "Standard for Interchangeability and Performance of Resistive Bridge Type Blood Pressure Transducers (Draft)," AAMI BPT-R (2/84) (February 1984 Revision) (Arlington, VA). A manufacturer may impose and/or use even more stringent standards. For example, the maximum fluid displacement (outward bulging of the diaphragm in the dome, e.g.) is severely restricted. Hence, potentially large pressure fluctuations must be accurately monitored over an extremely small range of displacement of a tube wall or diaphragm. Also, severe electrical constraints are placed on the equipment, such as the sensor; strict limits on electrical power levels on all devices near a patient must be observed. Further, the devices must be electrically isolated from the patients. Finally, as a practical matter, the transducer must provide repeatable and reliable measurements, particularly in view of the life-dependant nature of the environment in which they may be used.
Accordingly, it is one object of the present invention to provide a transducer for measuring bodily fluid pressure within a conduit which is low in cost of manufacture yet is of sufficient reliability to meet or exceed the restrictions placed thereon in the medical environment.
A further object of the present invention is to provide a transducer for measuring pressure within a conduit which provides a repeatable and reliable measurement without costly compensation for the effects of the temperature or the like.
An even further objective of the present invention is to replace the two part dome/sensor system with a one part transducer for measuring pressure within a conduit.
A still further objective of the present invention is to provide a transducer for measuring pressure within a conduit which is coupled to electronic circuitry adapted to generate an electrical signal compatible with currently available monitors.
Of course, many installations may prefer to retain the two part dome/sensor approach. Accordingly, a yet further object of the present invention is to provide an improved transducer for measuring the pressure within a conduit which is compatible with a two part dome/sensor system or the like and which has the above advantages without use of a strain gauge.